The Impala was Chevrolet’s most expensive passenger model through 1965, and had become the best-selling automobile in the United States.
The Impala name was first used for the full-sized 1956 General Motors Motorama show car that bore Corvette-like design cues, especially the grille. Painted emerald green metallic, with a white interior, the Impala featured hardtop styling. Clare MacKichan’s design team, along with designers from Pontiac, started to establish basic packaging and dimensions for their shared 1958 General Motors “A” body in June. The first styling sketch that would directly influence the finished Chevrolet automobile was seen by General Motors Styling vice president Harley Earl in October. Seven months later, the basic design was developed. (https://en.wikipedia.org/wiki/Chevrolet_Impala)